The present invention relates to an automatic focus-adjusting device, especially for use in still cameras.
An automatic focusing camera such that, when a photographer aims the camera at a subject and presses a shutter releasing button, the camera measures the distance to the subject, adjusts the lens so as to bring the subject into focus, and then exposes a photofilm has long been sought after.
A wide variety of methods for realizing such results have been proposed. In 1977, the KONIKA C35AF Camera (manufactured by Konishiroku Photo Ind. Co., Ltd., Japan) served as the springboard for a wide variety of automatic focusing cameras entering the market. However, such devices have not yet materialized. One of the reasons for not developing automatic focusing devices is that the reliability of a signal from a range finder for indicating the distance at which to focus has not been satisfactory.
This problem remains unsolved even at the present time when a number of automatic focusing cameras are being widely marketed. One method of solving this problem is to improve the reliability of range finders. However, it is impossible to obtain a range finder that measures subject and background distance without fail in every photographic situation.
For example, in the case of measuring a distance in reflected natural light from a subject (hereinafter referred to as "passive type measurement"), the reliability of a signal for measuring distance is lowered when the luminance of a subject is low. Distance is also measured by emitting light from a carmera and measuring the reflected light therefrom (hereinafter referred to as "active type measurement"). In an active type system, however, it is also impossible to avoid defects such as those that are introduced by distance itself. A signal for measuring a long distance is less reliable because the level of the signal is inversely proportional to the square of the distance being measured and the S/N ratio is lowered when the subject field is bright.
Another method of overcoming the defects is to utilize the depth of focus of a lens so as to compensate for the decrease in signal reliability.
Automatic focusing devices having been prompted by conventional means include those in which: a lens is set at the fixed focal point thereof when the depth of field of a diaphragm covers the closest distance to infinity; a lens is set at the hyper-focal point and an automatic focusing device is operated when a focusing distance is not farther than half of the hyper-focal distance of the lens, but the lens is set at the hyper-focal point when the focusing distance is otherwise; and a lens is set at the fixed focal point thereof when a signal given from a range finder device cannot serve as a signal for operating an automatic focusing device.
These proposals have peculiar effects by themselves and are used to improve the reliability of an automatic focusing device. However, there are problems of putting them to practical use because they each have respective defects.